This application claims priority to German application serial number 198 59 651.0, filed Dec. 15, 1998.
The invention concerns generally to dual-chamber cardiac pacemakers, and more particularly to dual-chamber pacemakers having both atrium-synchronous and asynchronous modes of operation.
Dual-chamber cardiac pacemakers which stimulate the ventricle synchronously with respect to atrial events are well-known in the art. Often these pacemakers are dual-mode and have a mode-switch function that switches the pacemaker from an atrium-synchronous to an asynchronous mode of operation, in the event that an upper limit rate of detected atrial cardiac actions is exceeded.
The reverse procedure, namely changing from an asynchronous to an atrium-synchronous mode of operation, i.e., termination of mode-switch episodes, is normally triggered by the detection of what is known as the xe2x80x98x out of y-criterionxe2x80x99. The xe2x80x98x out of y-criterionxe2x80x99 occurs when the number of atrially detected events within a predetermined total number of atrial events occur at a rate below the upper atrial limit rate.
However, two serious errors can occur in re-synchronization of the ventricular stimulation pulses to the atrially detected events. Firstly, the atrially detected events can result from the retrograde transfer of ventricular stimulation pulses. In that case, each ventricularly stimulated event causes a retrograde-transfer atrial event, which in turn triggers an AV-time, after which there is simulated a ventricular event which again causes a retrograde-transfer atrial event, and so forth. In such a case, re-synchronization to the atrially detected events would result in synchronization to the ventricle stimuli and directly trigger pacemaker mediated tachycardia (PMT).
Secondly, the atrially detected rate can result from a 2:1-detection of atrial events on the basis of atrial blanking, which makes each second atrial event xe2x80x98invisiblexe2x80x99 for the pacemaker. In such a case, re-synchronization would then directly result in the 2:1-block behaviour which is highly undesirable from a physiological point of view.
Attempts have been made to design around the re-synchronization problem. For example, U.S. Pat. No. 5,549,648 describes a pacemaker system and an operating procedure for the improved detection of the end phase of a retrograde transfer. The aim of the ""648 device is to provide a system capable of analyzing atrial and ventricular cardiac actions on a beat-to-beat basis by reference to given criteria (referred to as xe2x80x98beat-to-beatxe2x80x99 analysis). On the basis of that analysis, the operation of the pacemaker is then to be re-synchronized. However, the re-synchronization errors described above are not eliminated with the procedure of the ""648 patent. On the contrary, the system disclosed in the ""648 patent is only concerned with the fastest possible detection of the condition in which the mode switch episode can be concluded. Indeed, the aim of the ""648 patent, to expediently initiate re-synchronization, tends to make checking the analysis result and eliminating re-synchronization errors impossible.
Accordingly, a need exists for a dual-chamber cardiac pacemaker capable switching back from an asynchronous to an atrium-synchronous mode and re-synchronization of the pacemaker to the atrium in such a way as to substantially exclude malfunctions.
The present invention is direct to a dual-chamber pacemaker designed to avoid errors in the switching-back and re-synchronization procedures. Generally, the dual-chamber pacemaker comprises: a programming device, a body sensor, an atrium electrode, and a ventricle electrode. An internal telemetry may be further provided for connecting the implanted pacemaker to the programming device and a stimulation control unit may be provided for operational control. In such an embodiment, the stimulation control unit generally comprises an internal program memory, a data memory, and a clock and timer.
Any suitable body sensor may be utilized in the current invention. For example, the body sensor may include one or more of the following: a piezoelectric activity sensor, a sensor for blood oxygen saturation or blood temperature, or an impedance plethysmograph.
In one embodiment, a ventricle sensing unit and a ventricle stimulation unit are connected to the ventricle electrode, and similarly an atrium sensing unit and an atrium stimulation unit are connected to the atrium electrode. In such an embodiment, the sensing units may be connected to data inputs of the stimulation control unit and the stimulation units may be connected to control signal outputs thereof.
In another embodiment, the invention is also directed to a method of utilizing the dual-chamber pacemaker. In such an embodiment, the method of operation comprised the following steps: a) a counter used to detect fulfilment of the x-out of-y-criterion (xe2x80x98x-out of-y-counterxe2x80x99) is incremented only at the occurrence of certain known sequences of cardiac events (hereinafter also referred to as xe2x80x98signal sequence patternsxe2x80x99); b) if the counter condition satisfies a predetermined x-out of-y-criterion, in particular in the form of modulation or variation of a plurality of ventricularly detected or stimulated events (V), such as a V-V-intervalxe2x80x94a test is carried out for the presence of a retrograde transfer. If the interval between the V and atrially detected events (As), or V-AS, respectively following modulation are of a constant duration, the situation entails retrograde transfer. If, in contrast, the moment in time at which atrially detected events occur is independent of the (varied) moment in time of ventricular stimulation, the events were not retrogradedly transferred.
In yet another alternative embodiment, the event sequences detected for include: As-V-As wherein As-As greater than 1.000 ms; V-As-As-V wherein As-As greater than 1.000 ms; V-V-As wherein V-As greater than 1.000 ms; and V-V without interposed As.
In still another alternative embodiment, modulation is desirably implemented in the form of a prolongation of the time intervals by a post-ventricular atrial blanking time. In such an embodiment an additional increment and re-synchronization is activated if at least three successive V-As-time intervals were constant after modulated ventricle stimuli.
In still yet another embodiment of the invention, the criterion memory is adapted for the storage of at least one rate threshold value concerning the time intervals between successive atrial activitiesxe2x80x94preferably for the storage of two different threshold values in different stages of the test procedurexe2x80x94and the processing unit has a calculating unit for ascertaining the current atrial time intervals or atrial rate and (at least) one rate comparison unit for comparison thereof with the stored rate threshold value. The result of the rate comparison is then passed to a logic stage, together with the result of the x-out of-y-counting operation. In such an embodiment, activation of the switching means for switching over into the synchronous mode and re-synchronization of ventricle stimulation is made dependent not only on the result of the logical processing operation, but presupposing attainment of the counter condition xe2x80x98xxe2x80x99 by the counter means, on the additional presence of an output signal from the rate comparison unit which indicates when the atrial rate falls below a predetermined threshold value (xe2x80x98decision ratexe2x80x99). In accordance with the foregoing, in one such embodiment, the logic stage implements AND-gating of the x-out of-y-counting procedure and the additional test by modulation of the W-stimulation interval, which is preceded in particular also by OR-gating between the test result and the rate comparison.
In still yet another embodiment, the stimulation control unit desirably effects dynamic rate limitation, i.e., the ventricular stimulation rate, for avoiding PMT (xe2x80x98dynamic PMT limitxe2x80x99). In connection with such an embodiment, the stimulation function is further designed such that the specific rate limit value is selected based on the result of the test steps provided: 1) if in the outcome of the tests pacemaker operation is set to atrial sine rhythm (spontaneous atrial rate greater than sensor-indicated rate), the rate limitation should be based on the rate of spontaneous atrial activity (xe2x80x98sine ratexe2x80x99); and 2) if, however, the preceding tachycardia leads into atrial bradycardia (spontaneous atrial rate less than sensor-indicated rate), the limit rate should be derived from the sensor-indicated rate. These conditions also apply where retrograde transfer was established and active re-synchronization was implemented.